West Nile fever is a systemic acute fever disease caused by infection with West Nile virus (WNV). Occasionally, the virus invades and grows in the central nervous system to cause lethal brain meningitis. WNV is widely distributed in Africa, Middle East, part of Europe, Russia, India, Indonesia and the like. The virus is maintained and propagated by an infection ring between Culex species as a vector and birds (wild and domestic) as an amplification animal. During the process, human, horse and domestic animals become accidental hosts.
In summer 1999, WNV invaded and was indigenized in New York, USA, and has continuously been expanding since then. It was confirmed that more than 2300 persons were infected by the end of September last year (2007) throughout the United States, thus causing a serious problem for the public health. A WNV vaccine for human does not exist in the world at present.
Under the circumstances, propagation of the virus to Asian countries including Japan has been feared, and practicalization of human vaccine has been desired. while culture Vero cell-derived virus inactivation vaccines are being urgently developed at present, there is an increasing need for the development of a subunit vaccine that can be produced safely at a low cost without using a biosafety level 3 virus in the production step.
WNV was first isolated in the West Nile region of Uganda, Africa, in 1937 and is classified to belong to the Flaviviridae family falvivirus genus (non-patent document 1). The structure of virus particles consists of a spherical structure wherein a capsid protein (C protein) is bonded to one (+) chain RNA virus gene, and a lipid bilayer membrane surrounding the spherical structure. The lipid membrane includes two kinds of proteins: envelope protein (E protein) and membrane protein (M protein). M protein is produced as a precursor prM protein and cleaved with a protease called furin to become a mature protein.
The present inventors previously reported a production method of JEV VLP, comprising introducing an expression vector containing a cDNA fragment encoding prM protein and E protein of Japanese encephalitis virus (JEV) into an animal cell, culturing the obtained transformed cell and harvesting virus-like particles (VLP) secreted in the medium (patent document 1). However, no report has ever been made relating to the production of WNV subunit vaccine.
patent document 1: JP-A-2004-65118
non-patent document 1: Agrawal, A. G., L. R. Petersen, J. Infect. Dis., 188: 1-4 (2003)